soc: qcom: rpmpd: Add rpm power domains for msm8976

The MSM8956/76 SoCs have two main voltage-level power domains, VDD_CX
and VDD_MX, which also have their own voltage-floor-level (VFL)
corner.

Signed-off-by: AngeloGioacchino Del Regno <kholk11@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>